1. Field of the Invention
The present invention relates to a projection exposure apparatus, and more particularly, to a projection exposure apparatus having an active vibration isolator and a method of controlling vibration by the active vibration isolator. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for suppressing vibration caused by the movement of stages in the projection exposure apparatus.
2. Discussion of the Related Art
As a higher precision is desired in high-precision instruments such as a step-and-scan type exposure apparatus (so-called xe2x80x9cstepperxe2x80x9d), it has been necessary to isolate micro-vibrations generated on the base plate (or vibration isolating stand) of the apparatus from the installation floor at the micro-G level (very small level). Conventionally, mechanical dampers and pneumatic dampers have been used as vibration isolating pads to support the vibration isolating stand in the vibration isolators. The vibration isolating pads also have a function for centering the vibration isolators. The mechanical dampers consist of coil springs placed in a damping liquid. Pneumatic dampers may include an air spring. When a spring constant of the air spring is set at a small value, the vibrations exceeding approximately 10 Hz can be isolated. Accordingly, such vibration isolators are widely used for the support of precision instruments.
Recently, active vibration isolators have been proposed in order to overcome the inherent limits in the aforementioned passive vibration isolators. The active vibration isolators detect the vibration of the vibration isolating stand by sensors, and control the vibration by driving actuators based on the output of the sensors. The active vibration isolators thus are capable of providing an effective vibration isolation without significant vibrations in the low-frequency control region.
In conventional steppers, an XY stage (wafer stage) undergoing a large acceleration and deceleration is mounted on the base plate held by the vibration isolating pads. When the XY stage moves, the reaction force accompanying with the acceleration and deceleration of the XY stage causes the main body of the exposure apparatus to vibrate. To compensate for the vibration, in the active vibration isolator, a force having the same magnitude in the opposite direction as the reaction force accompanying the acceleration or deceleration of the stage is inputted in advance. In this case, six accelerometers measuring a movement in six degrees of freedom are attached to the main body of the exposure apparatus. Since a counterforce is applied during acceleration of deceleration of the stages, a mechanical resonance is excited in the main body of exposure apparatus so that the main body will vibrate at this frequency.
Moreover, when the amount of movement of the stage becomes larger because of the large acceleration or mass of the stage, vibration in the main body of the exposure apparatus (mainly in the rotational direction with respect to the Z-axis) changes the initial stage position. As a result, the amount of movement in the acceleration and mass of the stage will become even larger in successive operations. Accordingly, the amounts of vibration will lead to the failure of the equipment eventually when the vibration is not controlled precisely.
Accordingly, the present invention is directed to a projection exposure apparatus having an active vibration isolator and a method of controlling vibration by the active vibration isolator that substantially obviate one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a projection exposure apparatus having an active vibration isolator and a method of controlling vibration by the active vibration isolator regardless of the stage movement.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the penned drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a projection exposure apparatus transferring a pattern on a reticle to a wafer includes at least three vibration isolating pads, a vibration isolating stand on the vibration stand, a plurality of actuators driving the vibration isolating stand in a vertical direction, at least two stages over the vibration isolating stand, the stages mounting the wafer and the reticle, at least one displacement sensor at the vibration isolating stand and detecting a displacement of the vibration isolating stand, at least one acceleration sensor at the vibration isolating stand and detecting an acceleration and deceleration of the vibration isolating stand, a vibration control system coupled to the displacement sensors and the acceleration sensors, and controlling the actuators based on outputs of the displacement sensors and the acceleration sensor, a position measuring device positioned on the vibration isolating pads and measuring a position of the stage, and a vibration compensating system coupled to the vibration control system and receiving a command value to cancel a reaction force caused by the acceleration and deceleration of the stages in advance.
In another aspect of the present invention, a projection exposure apparatus transferring a pattern on a reticle to a wafer, the projection exposure apparatus having an active vibration isolator to isolate the apparatus from an vibration, the apparatus includes a plurality of isolation pads supporting the projection exposure apparatus, a vibration isolating stand on the isolation pads, a plurality of actuators driving the vibration isolating stand in a vertical direction, the actuators being below the vibration isolating stand, a plurality of stages mounting the wafer and reticle over the vibration isolating stand, a plurality of laser interferometer coupled to the stages and monitoring movements of the substrate and reticle, a plurality of acceleration sensors detecting an acceleration of the vibration isolating stand, the acceleration sensors positioned at the vibration isolating stand, a plurality of metal plates at the vibration isolating stand, a plurality of displacement sensors detecting a displacement of the vibration isolating stand, the displacement sensors facing into the metal plates, and an actuator control system controlling the actuators based on output signals from the displacement sensors and the acceleration sensors.
In a further aspect of the present invention, a method of controlling a vibration in a projection exposure apparatus, includes converting output signals from a plurality of displacement sensors and acceleration sensors into a displacement and acceleration in a center of gravity of the projection exposure apparatus with a plurality of degrees of freedom, respectively, calculating positional deviations and speed deviations of the projection exposure apparatus based on the displacement and acceleration, controlling a plurality of actuators through positional and speed controllers based on the positional and speed deviations, and inputting command values in advance into the positional and speed controllers to cancel a reaction force generated by a movement of wafer and reticle stages.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.